1. Field of the Invention
The present invention relates to an analog TV (television) broadcast signal receiving apparatus and an analog TV broadcast signal demodulating apparatus for use therewith, the analog TV broadcast signal receiving apparatus being designed illustratively to deal with diverse analog TV broadcasts used all over the world.
2. Description of the Related Art
A number of analog TV broadcast systems exist throughout the world. They differ from one another in terms of broadcast bandwidths and in frequency assignments of the picture and sound carriers involved. More specifically, there are three principal broadcast bandwidths: 6 (4.2) MHz, 7 (5) MHz, and 8 (6) MHz (the numbers in parentheses denote picture signal bandwidths).
The 6 MHz band is the broadcast band for the analog TV system used mainly in the United States. The 7/8 MHz band is the broadcast band for the analog TV system employed primarily in the European Union (Europe).
On ordinary broadcast bands (RF bands), the sound carrier is assigned a higher frequency than the picture carrier. In Germany, for example, Channel 2 has its picture and sound carrier frequencies appropriated at 48.25 MHz and 53.75 MHz, respectively.
In some regions of France, however, the picture carrier is assigned a higher frequency than the sound carrier. For example, the station called Channel A has its picture and sound carrier frequencies appropriated at 47.75 MHz and 41.25 MHz, respectively.
Except for the different frequency assignments of its picture and sound carriers, however, the above-mentioned French analog TV broadcast system has the same picture signal bandwidth and the same frequency interval between the picture and the sound carriers as the analog TV broadcast systems in the other regions. The channel of which the picture signal is assigned a higher frequency than the sound carrier is commonly referred to as Channel L′.
There have been attempts to devise analog TV broadcast signal receiving apparatuses capable of demodulating the TV signals of all the analog TV broadcast systems outlined above. Generally, these analog TV broadcast signal receiving apparatuses have their filters arranged to limit the picture and sound signal bands on the IF (intermediate frequency) band for each of the broadcast bandwidths involved, and are each furnished with mechanisms to switch between such band limitation filters.
More specifically, the vestigial sideband (VSB) filter for limiting the picture signal band and the band limitation filter for limiting the sound signal band are switched in keeping with the picture and sound carriers on each broadcast band. Generally, surface acoustic wave (SAW) filters are used to serve as the VSB filter for limiting the picture signal band and as the filter for limiting the sound signal.
FIG. 9 schematically shows a typical structure of an ordinary analog TV broadcast signal receiving apparatus designed for use in the United States and in the European Union. In FIG. 9, an analog TV broadcast signal received by a receiving antenna 101 is shown forwarded to an RF tuner 102. In the RF tuner 102, the received analog TV broadcast signal (i.e., RF signal) is converted in frequency to an IF signal TVif by a mixer 104 using a frequency signal coming from a variable frequency oscillator 103. The IF signal TVif is output by the RF tuner 102 by way of an amplifier 105.
FIGS. 10A, 10B and 10C schematically show how the IF signal TVif behaves on the 6, 7, and 8 MHz bands, respectively. FIG. 10A indicates the IF signal on the 6 MHz band used mainly in the United Sates. FIG. 10B represents the IF signal on the 7 MHz band employed primarily in Europe. FIG. 10C denotes the IF signal on the 8 MHz band utilized also in Europe. As shown in these figures, the IF signal differs from one band to another. That means each IF signal needs to have the appropriate SAW filter selected for use therewith.
The SAW filter selection is accomplished by first feeding the IF signal TVif from the RF tuner 102 to VSB filters (SAW filters) 106 and 107 for picture signal use and to sound selection SAW filters 108 and 109 for sound signal use. The VSB filter 106 is a SAW filter for use on the 6 MHz band (4.2 MHz for the picture signal band); the VSB filter 107 is a SAW filter used on the 7/8 MHz band (5/6 MHz for the picture signal band); the sound selection SAW filter 108 is utilized on the 6 MHz band; and the sound selection SAW filter 109 is employed on the 7/8 MHz band.
The outputs of the VSB filters 106 and 107 are supplied to a selector 110. The outputs of the sound selection SAW filters 108 and 109 are fed to a selector 111. The selectors 110 and 111 are controlled selectively by a host control section 130 providing overall control on the receiving apparatus as a whole.
The host control section 130 is composed of a microcomputer typically called a TV microcomputer. For each of the different markets (e.g., U.S., Europe) for which the receiving apparatus is destined, destination-oriented settings are registered in the host control section 130.
The host control section 130 receives user operation input information from a remote control transmitter 137. The user operation input information includes broadcast channel selection operation information. Upon receipt of the broadcast channel selection operation information from the remote control transmitter 137, the host control section 130 generates a selection control signal reflecting the destination-oriented setting information that has been registered inside and forwards the generated selection control signal to the variable frequency oscillator 103. In turn, the TV broadcast signal of the selected broadcast channel is converted to the corresponding IF signal TVif.
The host control section 130 also supplies the selectors 110 and 111 with selection signals in keeping with the destination-oriented setting information that has been registered in this receiving apparatus. Illustratively, if the receiving apparatus is set to receive the analog TV broadcast signal in the United States, then the microcomputer 130 causes the selector 110 to select the VSB filter 106 for the 6 MHz band. Similarly, if the receiving apparatus is arranged to receive the analog TV broadcast signal in the European Union, then the microcomputer 130 causes the selector 110 to select the VSB filter 107 for the 7/8 MHz band. The selector 110 outputs a picture IF signal Vif having undergone VSB demodulation.
Meanwhile, each sound signal requires that the applicable SAW filter be used to remove the picture signal from the IF signal TVif. For example, if the receiving apparatus is set to receive the analog TV broadcast signal on the 6 MHz band, the microcomputer 130 causes the selector 110 to select the sound selection SAW filter 108 for the 6 MHz band so as to remove the picture signal. Likewise, if the receiving apparatus is designed to receive the analog TV broadcast signal on the 7/8 MHz band, then the microcomputer 130 causes the selector 110 to select the sound selection SAW filter 109 for the 7/8 MHz band in order to eliminate the picture signal.
The picture IF signal Vif from the selector 110 and a sound band signal Sa from the selector 111 are supplied to an analog TV demodulation circuit 120. This analog TV demodulation circuit 120 is constituted by an LSI (large scale integrated circuit).
In the analog TV demodulation circuit 120, the picture IF signal Vif is fed to a multiplier 122 via an amplifier 121. The multiplier 122 multiplies the supplied picture IF signal by a frequency signal having the picture carrier frequency coming from a variable frequency oscillator 123. Although not shown, the variable frequency oscillator 123 is controlled by control signals from the host control section 130 in such a manner that the oscillation frequency of the oscillator 123 stays equal to the picture carrier frequency of the analog TV broadcast desired to be received.
A multiplication output signal from the multiplier 122 and the frequency signal from the variable frequency oscillator 123 are sent to a phase comparator 124. In turn, the phase comparator 124 generates a control signal reflecting the detected phase error between the two inputs and feeds the generated control signal to the variable frequency oscillator 123. These arrangements constitute a phase-locked loop (PLL).
In the PLL, the variable frequency oscillator 123 is controlled to output its frequency signal in synchronism with the picture carrier frequency. As a result, the multiplier 122 provides a baseband signal obtained by converting the picture carrier frequency of the picture IF signal Vif resulting from VSB demodulation into the baseband.
The baseband signal of the picture signal passes through a low-pass filter 125 that removes unnecessary signal components and is output by the analog TV demodulation circuit 120 as its output picture signal. This output picture signal is supplied to a video processor 131.
The video processor 131 performs color signal processing and other processes. The picture signal processed by the video processor 131 is fed to an LCD (liquid crystal display) panel 132 through an LCD panel driver, not shown. The picture signal is then output as a picture on the display screen of the LCD panel 132.
Meanwhile, the sound band signal Sa from the selector 111 is sent to a multiplier 127 via an amplifier 126 in the analog TV demodulation circuit 120. The frequency signal of the sound carrier frequency from a variable frequency oscillator 128 is also fed to the multiplier 127 and multiplied therein by the sound band signal Sa. Although not shown, the variable frequency oscillator 128 is controlled by control signals from the host control section 130 in such a manner that the oscillation frequency of the oscillator 128 stays equal to the sound carrier frequency of the analog TV broadcast desired to be received. The variable frequency oscillator 128, controlled in frequency by the control signal coming from the phase comparator 124, outputs the frequency signal in synchronism with the sound carrier frequency.
In the manner outlined above, the multiplier 127 converts the sound band signal Sa into the sound signal on the 4 to 6 MHz band. The 4 to 6 MHz band sound signal from the multiplier 127 is submitted to a band-pass filter 129 for removal of unnecessary signal components. Thereafter the sound signal from the band-pass filter 129 is output to a sound demodulation circuit 133. The sound demodulation circuit 133, also constituted by an LSI, demodulates the input sound signal into sounds that are supplied to and output acoustically by speakers 134.
As mentioned above, the signal L′ used in parts of France is configured such that the way its picture and sound carriers are assigned in inverse relation with each other in frequency as opposed to the carriers of the signals employed elsewhere. The reversed carrier assignments require having the SAW filters switched accordingly. A typical structure of a traditional receiving apparatus that takes these specificities into account is shown schematically in FIG. 11. The difference of the structure of FIG. 11 from that of FIG. 9 is that the VSB filter 106 for 6 MHz and the sound selection SAW filter 109 for 6 MHz are removed and replaced simply by a sound SAW filter 135 for the signal L′. For the related art regarding the present information, reference should be made to Japanese Patent Laid-Open No. 2006-191388.